1. Field of the Invention
The present invention relates to a projection exposure apparatus and, more particularly, to a projection exposure apparatus in a lithographic process of semiconductor integrated circuits.
2. Related Background Art
As a conventional projection exposure apparatus, as described in U.S. Pat. No. 4,871,237, known is a projection exposure apparatus (stepper) wherein an internal pressure between lens elements of a projection lens constituted by a plurality of lenses is changed to change the magnification of a projected image, and exposure is performed in consideration of a pattern strain caused by changes in atmosphere (e.g., an atmospheric pressure, a temperature, and a humidity) and expansion/shrinkage of wafers.
When a plurality of projection exposure apparatuses are assumed to be used, they are adjusted so that image strains between the plurality of projection exposure apparatuses are averaged and optimized so as to prevent pattern alignment errors regardless of various combinations of use of the plurality of projection exposure apparatuses.
In order to improve overlay accuracy of layers exposed by different projection exposure apparatuses, as disclosed in U.S. Pat. No. 4,734,746, there is proposed a method of adjusting relative magnifications of the respective apparatuses so as to optimize the overlay accuracy of the images projected in accordance with image strain characteristics of the respective apparatuses. According to this method, when a characteristic curve (aberration curve) associated with an image strain in a predetermined exposure range from the center (optical axis) of the field of view of the first projection exposure apparatus to a predetermined image height position is compared with that of the second projection exposure apparatus used for overlay exposure while projection magnifications of projection optical systems of these apparatuses are changed relative to each other, absolute values of differences between image strain amounts obtained at a plurality of arbitrary image height positions within the predetermined exposure range from the optical axis of the first projection exposure apparatus to a predetermined image height position and image strain amounts within the predetermined exposure range of the second projection exposure apparatus are obtained. The projection magnification of at least one of the first and second projection exposure apparatuses is adjusted so that a maximum one of the absolute values is set to be a minimum value. As a magnification correcting means, some of the distances between lens elements constituting the projection optical system are sealed from the outer air, and internal air pressures are changed. According to an alternative method, some lens elements are vertically moved in the direction of the optical axis.
Although a conventional projection exposure apparatus has a means for correcting an image strain of a projection optical system, its operation aims at mainly preventing variations in image strain in accordance with changes in atmospheric conditions or eliminating correctable residual errors during assembly adjustment.
In the conventional technique described above, however, isotropic components (i.e., components symmetric about the optical axis) of the magnification and distortion characteristics can only be controlled. For this reason, when overlay exposure is to be performed for a wafer anisotropically deformed during annealing or the like in the IC manufacture or a wafer printed in an anisotropic image strain state during exposure of the immediately preceding layer, sufficiently high overlay accuracy between a projected image and a pattern on a wafer cannot be obtained by isotropic correction as in the conventional technique. In addition, in the projection optical system arranged in the projection exposure apparatus, overlay errors inevitably occur in an image strain which cannot be eliminated by adjustment since this image strain occurs due to small machining errors during the manufacture of optical members or during assembly. In a conventional system using a plurality of apparatuses, adjustment is performed to minimize these errors in all combinations of the apparatuses. For example, assume that apparatuses A, B, and C are used. When characteristics of the apparatus A are matched with those of the apparatus B, overlay accuracy between the apparatuses A and C is degraded. It is therefore difficult to strictly match the characteristics between all the apparatuses.
Even if overlay accuracy is to be improved by adjusting magnifications of different apparatuses, only isotropic magnification correction with respect to the optical axis is available in the prior arts. When optical axes of the apparatuses are offset from each other, or an asymmetric (anisotropic) image strain with respect to the optical axis is present, the above correction does not necessarily serve as an effective means.